Boron is one of the essential elements for proper plant growth. Boron deficiency in plants, such as citrus, causes curling of the leaf, embrittlement and brown coloration, resulting in a heavy shedding of young fruit. The element is required in relatively small quantities and is known as micronutrient along with some other essential elements such as zinc, manganese, copper, iron and molybdenum. The conventional method employed to combat boron deficiency is to apply an aqueous solution of borax--as much as 2 lb. per tree is used--which is wasteful because it is readily washed away by water.
The art is aware of many oil soluble boron compounds which heretofore have been used as additives in gasoline and lubricating oils. Organic esters of boric acid are used widely for this purpose. However, hydrolytic stability is an important property both in the storage and application of oils, and in general organic borates are poor in this respect. Certain organic esters of boric acid are more hydrolytically stable than others because of their special spacial arrangement which hinders hydrolytic fission. Triethanolamine borate is such a compound (Brown and Fletcher, J. Amer. Chem. Soc., 1951, 73, 2808) because of its cage like (triptych) structure. Indeed Steinberg and Hunter (Ind. Eng. Chem., 1957, 49, 174) have shown that the relative rates of hydrolysis of the aliphatic and alicylic borates are in the order predicted by the steric requirements in the nucleophilic attack by water on the boron atom. For example tri(diisobutylcarbinyl)borate is 50% hydrolysed in water at 21.degree. C. only after 221 days. Borates with lipophilic groups, e.g., long chain alkyl, would also be expected to be relatively more hydrolytically stable. The more hydrolytically stable esters of boric acid and alkyl/aryl boronic acids include the cyclic esters, the alkanolamine esters, the non-alpha-hydrogen esters, and branched alkyl esters such as the tri-(methylisobutylcarbinyl)esters. Esters of polymeric hydroxyl compounds, e.g., the hydrolyzed copolymer of vinylacetate and C.sub.16-18 alkanes, described in British Pat. No. 828,634 (Chem. Abs., 1960, 54, 13728b), are also suitable. Oil solubility might be a restriction with some of the esters but can be overcome by using a mixed carrier oil as described later. Alternatively the solubility of the borate can be improved by forming an adduct with an oil solubility moiety.
It is a primary object of the instant invention to provide novel, hydrolytically, stable boron-containing spray oils for economically and and efficiently supplying boron to plants.
In conjunction with the above object, it is also a highly significant object of the invention to provide an improved process for supplying boron to plants by foliar application thereto.